Technical Abstract:
Triticum mosaic virus (TriMV) is the type species of the genus Poacevirus in the family Potyviridae. The genomic organization of TriMV is similar to that of the members of Potyvirus genus but has only limited sequence identity with other potyviruses. In this study, we screened the genome of TriMV for posttranscriptional gene silencing (PTGS) suppression activity by cloning individual cistrons in pCASS4, followed by coinfiltration assays in transgenic Nicotiana benthamiana line 16C leaves with agrobacterium harboring genes of interest and sense GFP mRNA. P1 was the only TriMV encoding region to exhibit suppression of PTGS activity induced either by sense GFP mRNA or by a GFP inverted repeat RNA. Furthermore, TriMV P1 suppressed the systemic silencing induced by sense GFP mRNA in 16C N. benthamiana plants. Mutational analyses of P1 cistron revealed that a 10 amino acid deletion at the C-terminal region completely abolished the silencing suppression, whereas a 40 amino acid deletion at the N-terminus had no effect on silencing suppression activity. Additionally, point mutations at conserved cystein and histidine residues in a putative zinc-finger domain affected PTGS suppression activity of P1 induced by sense GFP mRNA in 16C N. benthamiana leaves. Collectively, these data indicate that the P1 cistron of members of Poacevirus genus function in suppression of RNA silencing induced by PTGS similar to that of the P1 cistrons of Ipomovirus and Tritimovirus genera of Potyviridae, and thus differs with the members of Potyvirus genus, whose members encode HC-Pro as suppressor of PTGS.